In recent years, in order to increase the transmission capacity of an optical transmission system, an optical transmission system having a high bit rate higher than 40 Gbit/s has been and is being researched and developed.
In order to cope with rapid increase of the network traffic, further increase of the bit rate is essentially required. Particularly as an optical modulation system which achieves optical transmission of 50 Gbit/s or more, a quadrature phase shift keying (QPSK) system or a differential quadrature phase shift keying (DQPSK) system is considered most promising.
In order to demodulate signal light modulated by such a QPSK system or a DQPSK system as just described, a coherent optical receiver including a 90-degree hybrid is required. Here, the 90-degree hybrid exhibits output forms (patterns) having a different branching radio depending upon the phase modulation state of QPSK signal light or DQPSK signal light and is the most important component of a coherent optical receiver.
As conditions demanded for such a 90-degree hybrid as just described, low loss, a wide band characteristic property of an operating wavelength (low wavelength dependency), a low phase displacement characteristic, compactness, a monolithic integration characteristic and so forth can be listed.
At present, 90-degree hybrids which use a bulk component are placed on the market.
FIG. 48A is a view illustrating a general configuration of a 90-degree hybrid which uses a bulk component, and FIG. 48B is a phase relationship diagram illustrating a phase relationship of optical signals outputted from the 90-degree hybrid.
It is to be noted that reference characters S−L, S+L, S+jL and S−jL in FIG. 48A indicate what relative relationship the phase of local oscillation (LO) light (L) has with reference to the phase of signal light (S). Here, S−L and S+L indicate that they have a phase relationship displaced by 180 degrees from each other, and S+jL and S−jL indicate that they have a phase relationship displaced by 90 degrees with respect to S+L and S−L, respectively. Further, the phase relationship diagram of FIG. 48B illustrates a phase relationship of optical signals outputted from the 90-degree hybrid in response to a relative phase difference between the QPSK signal light and the LO light.
As seen in FIG. 48A, QPSK signal light and LO light are inputted to two input channels of the 90-degree hybrid. Then, optical signals having an in-phase relationship with each other are outputted from a first output channel (Ch-1) and a second output channel (Ch-2) from among four output channels. Meanwhile, optical signals having a quadrature phase relationship with the optical signals having the in-phase relationship are outputted from a third output channel (Ch-3) and a fourth output channel (Ch-4) from among the four output channels of the 90-degree hybrid.
Such a 90-degree hybrid formed using a bulk component as just described has such superior characteristics as low loss, a low wavelength dependency and a low phase deviation characteristic.
Meanwhile, also a 90-degree hybrid having an optical waveguide structure which can be monolithically integrated has been and is being researched and developed.
FIGS. 49A and 50A illustrate general configurations of 90-degree hybrids based on waveguide optics, and FIGS. 49B and 50B are phase relationship diagrams illustrating phase relationships of optical signals outputted from the 90-degree hybrids of FIGS. 49A and 50A, respectively.
It is to be noted that reference characters S−L, S+L, S+jL and S−jL in each of FIGS. 49A and 50A indicate what relative relationship the phase of LO light (L) has with reference to the phase of signal light (S). Here, S−L and S+L indicate that they have a phase relationship displaced by 180 degrees from each other, and S+jL and S−jL have a phase relationship displaced by 90 degrees with respect to S+L and S−L, respectively. Further, the phase relationship diagram of each of FIGS. 49B and 50B illustrates a phase relationship of optical signals outputted from the 90-degree hybrid in response to a relative phase difference between the QPSK signal light and the LO light.
First, the 90-degree hybrid illustrated in FIG. 49A is formed from four 3-dB couplers and a 90-degree phase shifter. A phase relationship of optical signals outputted from the 90-degree hybrid formed in this manner is similar to that of the 90-degree hybrid formed using a bulk component described hereinabove as illustrated in FIG. 49B.
The 90-degree hybrid having such a configuration as described above is suitable for monolithic integration and is expected to have a low wavelength dependency and a low phase displacement characteristic.
On the other hand, the 90-degree hybrid illustrated in FIG. 50A is formed from a 4:4 multimode interference (MMI) coupler having four channels on both of the input side and the output side thereof.
Here, in order to obtain 90-degree hybrid operation using a 4:4 MMI coupler, it is necessary to select two channels at asymmetrical positions from among four channels on the input side of the 4:4 MMI coupler as input channels for inputting QPSK signal light and LO light. With such selection, a relationship of phases different from each other by 90 degrees is obtained inevitably by mode interference in the MMI region of the 4:4 MMI coupler, and therefore, the 4:4 MMI coupler can be used as a 90-degree hybrid.
The 90-degree hybrid having such a configuration as described above is suitable for monolithic integration and is superior in that it can be configured compact.
However, a phase relationship of optical signals outputted from the 90-degree hybrid just described indicates rotation by approximately 45 degrees with respect to the phase relationships [refer to FIGS. 48B and 49B] of the 90-degree hybrids illustrated in FIGS. 48A and 49A as illustrated in FIG. 50B. This is because, when two input lights interfere in mode with each other, a phase difference by 45 degrees is produced inevitably.
Meanwhile, in the case of the 90-degree hybrid illustrated in FIG. 50A, a pair of optical signals having an in-phase relationship with each other are outputted from the two outer side channels (Ch-1 and Ch-4) while a pair of optical signal having a quadrature phase relationship with the pair of optical signals having the in-phase relationship are outputted from the two inner side channels (Ch-2 and Ch-3). In short, a pair of optical signals having an in-phase relationship with each other are outputted from two output channels (Ch-1 and Ch-4) spatially spaced away from each other.